The development of semiconductor devices and integrated circuits has been characterized by a continuing increase in the quality of the performance of the commercially available semiconductor devices which has been concurrent with steady increases in the number of active and passive devices formed on a semiconductor chip. As the geometries of individual active and passive devices are reduced, continuing innovation is required in order to develop and perfect processes which are employed in the manufacture of high density circuitry. Most high density circuitry, particularly of the type found in MOS random access memories, currently employs polycrystalline silicon (also known as polysilicon) as a conductive word line material. However, it has been found that as the geometry of the conductive polysilicon lines is reduced, the electrical resistance increases, thereby increasing the response time of the memory cells to any given signal. It is clear that what is needed is a material having better conductivity for the word lines.
Aluminum has been suggested. However, aluminum suffers from the drawback that layers of it have a tendency to dissolve into the surrounding silicon on the wafer and also have a tendency to melt and even vaporize when subsequent diffusion, deposition or annealing operations are carried out on the wafer.
Experimentation has been performed with depositing refractory metals such as molybdenum and tungsten, as well as molybdenum silicide and tungsten silicide. The metallic molybdenum and tungsten as well as the molybdenum silicide and tungsten silicide is usually deposited, as is aluminum, either through physical evaporation techniques by sputtering pure metal or co-sputtering metal with a silicon compound. Both the physical evaporation and sputtering methods have drawbacks, in that the equipment required to perform the operations is relatively expensive and the throughput from the expensive capital equipment is relatively low. In addition, both physical evaporation and sputtering provide poor step coverage on the wafer.
What is needed is a relatively inexpensive method of depositing refractory metals and particularly tungsten and in a manner similar to that which is currently employed to deposit silicon nitride, polysilicon and silicon oxides, that is, a chemical vapor deposition method.
Experimentation has been done to determine whether state of the art chemical vapor deposition methods can be used to deposit tungsten. Applicants have found that use of a hot wall reactor system with vertically oriented wafers, operating at low pressure, of the type disclosed in U.S. Pat. No. 3,900,597 to Chruma et al. and U.S. Pat. No. 4,279,947 to Goldman et al. does not yield suitable metal films for the standards of the microelectronics industry.
In particular, Applicants have operated a hot wall, low pressure, chemical vapor deposition system employing tungsten hexafluoride, WF.sub.6, as a reactant gas for the chemical vapor deposition of metallic tungsten. Applicants have found that process uniformity and repeatability are difficult, if not impossible, to control. Wafers located adjacent each other often had radically different tungsten metal film thicknesses. In addition, the deposited tungsten adhered very poorly to the silicon wafers. Applicants found that cellophane or adhesive tape pressed into adhesion with the layer of tungsten deposited in the hot wall tungsten hexafluoride process could peel the deposited tungsten layer off the wafer. Furthermore, Applicants found that it was impossible to compensate for reactant gas depletion effects due to consumption of the tungsten hexafluoride by the well known expedient of ramping or linearly increasing the temperature of the reactor tube along its length in order to maintain a constant pyrolysis rate.
Another problem which Applicants encountered in their examination of the hot wall process to deposit tungsten metal was unwanted gas phase nucleation and deposition of tungsten on the reactor tube walls. In other words, rather than the tungsten hexafluoride pyrolyzing to deposit tungsten only on the wafers, tungsten was being deposited throughout the interior of the reactor tube, creating a large amount of particulate matter which fell onto the wafers being processed and ruined them.
Thus, what is needed is a chemical vapor deposition apparatus and process for the deposition of metallic tungsten. The process must be uniform, repeatable, provide adhesive films of tungsten having a low resistance and be a relatively clean process.